Our objectives is to learn how receptors on lymphocyte membranes function in regulating cellular differentiation. We want to know the function of the antigen-binding receptor, whether ligands induce interaction with other types of membrane receptors, how the consequences of antigen binding are integrated with signals received from interacting cells, and how membrane organization governs receptor function. Our approach to the problem of receptor function combines two aspects: 1) developing chemical methods to visualize receptors, identify physical association or interaction of surface antigens with other membrane components, and isolate subpopulations of B and T cells on the basis of distinguishing surface markers; and 2) correlating the physical characterization of lymphocyte subsets with their function in a well-defined and readily manipulated in vitro anti-hapten antibody response. Our methods emphasize the use of hapten-sandwich procedures (hapten-conjugated antigens, antibodies, and cell surfaces in combination with anti-hapten antibodies) to induce the selective bridging of various receptors on the same or on different cells, to purify cells that bear particular receptors, and to deliver effectively biologically active molecules to selected cell populations.